The present invention is directed to a method of identifying the cylinder combustion sequence of a four-stroke spark ignition internal combustion engine, and more particularly to an identification method based on the detected ion current.
The ion current across a spark plug gap in an internal combustion engine is sometimes measured as an indication of the combustion strength. Typically, the measured ion current for each engine cylinder is integrated over a predetermined interval to form a combustion quality (CQ) indication that is compared to a threshold. A valid combustion event is verified if the CQ indication is above the threshold, whereas a misfire or non-combustion event is identified if the CQ indication is below the threshold.
One use of the CQ indication in four-stroke engines is to reliably resolve the ambiguity between cylinder stroke and crankshaft position to enable individual cylinder fuel and spark control. During initial engine operation, fuel is distributed to the various engine cylinders on a semi-random basis, and the spark plugs for a pair of opposing engine cylinders (i.e., combustion and exhaust) are fired together, relative to a reference crankshaft position. This initial spark control is commonly referred to as a waste spark mode since the spark discharge in the exhaust cylinder is wasted. If CQ indications above a calibrated threshold are generated in synchronism with an assumed combustion pattern, the assumed combustion pattern is deemed to be correct, and sequential fuel and spark controls are commenced. Otherwise, the assumed cylinder combustion sequence is deemed to be incorrect, and is adjusted by 180xc2x0 before transitioning to sequential fuel and spark control. See, for example, the Research Disclosure No. 41702, published in January 1999.
While the above-described control can quickly and reliably identify the correct combustion sequence under normal conditions, it has been found that in certain conditions, the CQ indications for both normal combustion and misfire events tend to be lower or higher than under normal conditions. For example, spark plug fouling or the presence of certain fuel additives tends to bias the CQ indications above the normal threshold, even with the incorrect cylinder combustion sequence. On the other hand, the CQ indications sometimes fall below the normal threshold just after engine starting, even with the correct cylinder combustion sequence. As a result, it can take an extended period of time to correctly identify the cylinder combustion sequence, and the reliability of the control is less than desired.
The present invention is directed to an improved method of identifying the cylinder combustion sequence of a four-stroke internal combustion engine relative to a reference engine position based on measured ion current combustion quality (CQ) indications, wherein an assumed combustion sequence is quickly and reliably accepted or rejected in any engine operating condition. According to the invention, individual CQ indications for the various engine cylinders are not compared to a threshold, but rather, are algebraically combined as a function of the assumed combustion sequence so that the combined CQ indication increases in a first polarity if the assumed combustion sequence is correct, and in a second polarity if the assumed combustion sequence is incorrect. When the absolute value of the combined CQ indication exceeds a threshold, the polarity of the combined CQ indication is used to either accept or reject the assumed combustion sequence. Each of the measured CQ indications are used without regard to the engine operating condition, and the threshold to which the combined CQ indication is compared is reflective of a confidence level in the assumed combustion sequence, and is not used to distinguish between combustion and exhaust strokes in an individual engine cylinder.
In a preferred embodiment, individual CQ indications associated with the assumed combustion strokes increase the combined CQ indication, whereas CQ indications associated with the exhaust strokes decrease the combined CQ indication. Since the exhaust stroke CQ indications are, on average, lower than the combustion stroke CQ indications for any given operating condition, the combined CQ indication will increase if the assumed combustion sequence is correct, and decrease if the assumed combustion sequence is incorrect.